Alternating starter use during multi-engine motoring

ABSTRACT

A system is provided for alternating starter use during multi-engine motoring in an aircraft. The system includes a first engine starting system of a first engine. A first controller is in communication with a second controller that controls a second engine starting system of a second engine, the first controller being configured to intermittently direct power to the first engine starting system to alternately accelerate and decelerate the first engine during motoring with respect to the second engine.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No. 15/652,692 filed Jul. 18, 2017, which claims priority to U.S. Provisional Application No. 62/365,097 filed Jul. 21, 2016, the disclosure of which is incorporated by reference herein in its entirety.

BACKGROUND

This disclosure relates to gas turbine engines, and more particularly to systems and methods for alternating starter use during multi-engine motoring of gas turbine engines.

Gas turbine engines are used in numerous applications, one of which is for providing thrust to an airplane. When the gas turbine engine of an airplane has been shut off for example, after an airplane has landed at an airport, the engine is hot and due to heat rise, the upper portions of the engine will be hotter than lower portions of the engine. When this occurs thermal expansion may cause deflection of components of the engine which may result in a “bowed rotor” condition. If a gas turbine engine is in such a bowed rotor condition it is undesirable to restart or start the engine.

One approach to mitigating a bowed rotor condition is to use a starter system to drive rotation of a spool within the engine for an extended period of time at a speed below which a resonance occurs (i.e., a critical speed or frequency) that may lead to damage when a sufficiently large bowed rotor condition is present. Motoring is typically performed separately for each engine at different times, which extends the total start time for a multi-engine aircraft.

BRIEF DESCRIPTION

In an embodiment, a system is provided for alternating starter use during multi-engine motoring in an aircraft. The system includes a first engine starting system of a first engine. A first controller is in communication with a second controller that controls a second engine starting system of a second engine, the first controller being configured to intermittently direct power to the first engine starting system to alternately accelerate and decelerate the first engine during motoring with respect to the second engine.

In addition to one or more of the features described above, or as an alternative to any of the foregoing embodiments, further embodiments may include where control of the first engine starting system is based on one or more of: an ambient air temperature, an ambient pressure, an oil temperature, engine drag, and parasitic factors.

In addition to one or more of the features described above, or as an alternative to any of the foregoing embodiments, further embodiments may include where a power source of the motoring includes a pneumatic or electric power source from at least one of: an auxiliary power unit of the aircraft, a ground cart, a cross engine supply, or a stored energy system.

In addition to one or more of the features described above, or as an alternative to any of the foregoing embodiments, further embodiments may include where control of the first engine starting system is based on one or more of: an engine speed, a starter system electric current, and a rate of starter system electric current change.

In addition to one or more of the features described above, or as an alternative to any of the foregoing embodiments, further embodiments may include where control of the first engine starting system is based on commanding one or more adjustments to a baseline dry motoring profile to shift timing of supply of the compressed air on an engine-by-engine basis.

In addition to one or more of the features described above, or as an alternative to any of the foregoing embodiments, further embodiments may include where the first controller is configured to coordinate with a second controller to control the first engine starting system and a second engine starting system of the second engine.

In addition to one or more of the features described above, or as an alternative to any of the foregoing embodiments, further embodiments may include where the first controller is configured to alternate supplying power from a power source between the first engine starting system and the second engine starting system.

According to an embodiment, a system of an aircraft includes a compressed air source operable to supply compressed air, a first engine system, and a second engine system. The first engine system includes a first gas turbine engine, a first air turbine starter, and a first controller operable to control delivery of the compressed air to the first air turbine starter during motoring of the first gas turbine engine. The second engine system includes a second gas turbine engine, a second air turbine starter, and a second controller operable to control delivery of the compressed air to the second air turbine starter during motoring of the second gas turbine engine. The system also includes a means for causing supply of the compressed air to alternate between the first air turbine starter and the second air turbine starter, and intermittently accelerate and decelerate the first gas turbine engine with respect to the second gas turbine engine.

In addition to one or more of the features described above, or as an alternative to any of the foregoing embodiments, further embodiments may include where the system is configured to coordinate control, via the first controller and the second controller, the alternating supply of the compressed air between the first air turbine starter and the second air turbine starter based on one or more of: an ambient air temperature, an ambient pressure, an oil temperature, engine drag, and parasitic factors.

In addition to one or more of the features described above, or as an alternative to any of the foregoing embodiments, further embodiments may include where the system is configured to coordinate control, via the first controller and the second controller, the alternating supply of the compressed air between the first air turbine starter and the second air turbine starter based on one or more of: an engine speed, a starter speed, a starter valve position, a starter air pressure, a rate of speed change, a rate of starter valve position change, and a rate of starter air pressure change.

In addition to one or more of the features described above, or as an alternative to any of the foregoing embodiments, further embodiments may include where the system is configured to coordinate control, via the first controller and the second controller, the alternating supply of the compressed air between the first air turbine starter and the second air turbine starter by commanding one or more adjustments to a baseline dry motoring profile to shift timing of use of the compressed air on an engine-by-engine basis.

In addition to one or more of the features described above, or as an alternative to any of the foregoing embodiments, further embodiments may include a first starter air valve configured to control the first air turbine starter and a second starter air valve configured to control the second air turbine starter.

In addition to one or more of the features described above, or as an alternative to any of the foregoing embodiments, further embodiments may include where the system is configured to coordinate control of the first starter air valve through the first controller and the second starter air valve through the second controller.

In addition to one or more of the features described above, or as an alternative to any of the foregoing embodiments, further embodiments may include where a first engine control interface is configured to buffer communication of the first engine system from aircraft level communication, a second engine control interface is configured to buffer communication of the second engine system from aircraft level communication, and the first engine control interface is configured to communicate with the second engine control interface.

Another embodiment includes a method for alternating starter use during multi-engine motoring in an aircraft. The method includes supplying, by a compressed air source, compressed air during motoring. The first controller commands the first starter air valve of a first gas turbine engine to open while a second starter air valve of a second gas turbine engine is closed, the first starter air valve supplying the compressed air to a first air turbine starter of the first gas turbine engine. The first controller commands the first starter air valve to at least partially close based on a first condition. A second controller commands the second starter air valve to open to supply the compressed air to a second air turbine starter of the second gas turbine engine after the first starter air valve is at least partially closed. The second controller commands the second starter air valve to at least partially close based on a second condition.

In addition to one or more of the features described above, or as an alternative to any of the foregoing embodiments, further embodiments may include where timing of respective commanding steps is based on one or more of: an ambient air temperature, an ambient pressure, an oil temperature, engine drag, and parasitic factors.

In addition to one or more of the features described above, or as an alternative to any of the foregoing embodiments, further embodiments may include commanding one or more adjustments to a baseline dry motoring profile to shift timing of supply of the compressed air on an engine-by-engine basis.

In addition to one or more of the features described above, or as an alternative to any of the foregoing embodiments, further embodiments may include where the first condition and the second condition are dynamically configurable.

In addition to one or more of the features described above, or as an alternative to any of the foregoing embodiments, further embodiments may include where supplying the compressed air to a first air turbine starter of the first gas turbine engine causes a first starting spool of the first gas turbine engine to accelerate, the first starting spool decelerates in response to at least partially closing the first starter air valve, a second starting spool of the second gas turbine engine accelerates in response to the opening of the second starter air valve, and the second starting spool decelerates in response to at least partially closing the second starter air valve.

In addition to one or more of the features described above, or as an alternative to any of the foregoing embodiments, further embodiments may include where the commanding steps are repeated until a motoring termination condition is met.

A technical effect of the apparatus, systems and methods is achieved by using alternating starter air use during coordinated multi-engine motoring for bowed rotor mitigation of gas turbine engines as described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter which is regarded as the present disclosure is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the present disclosure are apparent from the following detailed description taken in conjunction with the accompanying drawings in which:

FIG. 1 is a schematic illustration of an aircraft engine starting system in accordance with an embodiment of the disclosure;

FIG. 2 is another schematic illustration of an aircraft engine starting system in accordance with an embodiment of the disclosure;

FIG. 3 is a schematic illustration of a high spool gas path with a straddle-mounted spool in accordance with an embodiment of the disclosure;

FIG. 4 is a schematic illustration of a high spool gas path with an overhung spool in accordance with an embodiment of the disclosure;

FIG. 5 is a block diagram of a system for bowed rotor start mitigation in accordance with an embodiment of the disclosure;

FIG. 6 is a block diagram of multi-engine coordination command logic in accordance with an embodiment of the disclosure;

FIG. 7 depicts relative starter air valve timing for multiple starter air valves in accordance with an embodiment of the disclosure;

FIG. 8 depicts engine speed oscillations for multiple engines in accordance with an embodiment of the disclosure;

FIG. 9 is a flow chart illustrating a method in accordance with an embodiment of the disclosure; and

FIG. 10 is another schematic illustration of an aircraft engine starting system in accordance with an embodiment of the disclosure.

DETAILED DESCRIPTION

Various embodiments of the present disclosure are related to a bowed rotor start mitigation system in gas turbine engines. Embodiments can include using a starter air valve to control a rotor speed of a starting spool of a gas turbine engine to mitigate a bowed rotor condition using a dry motoring process for multiple engines. During dry motoring, the starter air valve can be actively adjusted to deliver air pressure (i.e., compressed air) from an air supply to an air turbine starter of an engine starting system that controls starting spool rotor speed. Dry motoring may be performed by running an engine starting system at a lower speed with a longer duration than typically used for engine starting while dynamically adjusting the starter air valve to maintain the rotor speed and/or follow a dry motoring profile. The critical rotor speed refers to a major resonance speed where, if the temperatures are unhomogenized, the combination of a bowed rotor and similarly bowed casing and the resonance would lead to high amplitude oscillation in the rotor and high rubbing of blade tips on one side of the rotor, especially in the high pressure compressor if the rotor is straddle-mounted.

A dry motoring profile for dry motoring can be selected based on various parameters, such as a modeled temperature value of the gas turbine engine used to estimate heat stored in the engine core when a start sequence is initiated and identify a risk of a bowed rotor. The modeled temperature value alone or in combination with other values (e.g., measured temperatures) can be used to calculate a bowed rotor risk parameter. For example, the modeled temperature can be adjusted relative to an ambient temperature when calculating the bowed rotor risk parameter. The bowed rotor risk parameter may be used to take a control action to mitigate the risk of starting the gas turbine engine with a bowed rotor. The control action can include dry motoring consistent with the dry motoring profile. In some embodiments, a targeted rotor speed profile of the dry motoring profile can be adjusted as dry motoring is performed. The dry motoring profile can be configured to oscillate the motoring speed in an alternating pattern between multiple engines such that rotor speed of each engine stays below the critical rotor speed while not completely stopping rotation during dry motoring. For instance, in a two engine system, the motoring systems alternate between receiving more compressed air and less compressed air in a staggered pattern such that one engine is accelerating while the other engine is decelerating to reduce loading on a compressed air source that drives the motoring of both engines. In some embodiments, each engine has a baseline dry motoring profile that can be time shifted on an engine basis to achieve an alternating pattern. Dry motoring profiles can be formatted on a rotor speed basis (e.g., N2), a starter speed basis (e.g., NS), a valve position basis, and/or using one or more other control parameters.

In alternate embodiments, electric starters are used in motoring systems to perform dry motoring. The electric starters alternate in drawing electric current from an electric power source such that one engine is accelerating while the other engine is decelerating to reduce loading on the electric power source that drives the motoring of both engines. Dry motoring profiles can be defined on an electrical current draw basis.

A full authority digital engine control (FADEC) system or other system may send a message to the cockpit to inform the crew of an extended time start time due to bowed rotor mitigation actions prior to completing an engine start sequence. If the engine is in a ground test or in a test stand, a message can be sent to the test stand or cockpit based on the control-calculated risk of a bowed rotor. A test stand crew can be alerted regarding a requirement to keep the starting spool of the engine to a speed below the known resonance speed of the rotor in order to homogenize the temperature of the rotor and the casings about the rotor which also are distorted by temperature non-uniformity.

In order to further reduce total start time, embodiments control dry motoring of multiple engines at the same time. Respective FADECs for each engine can provide parameters to one or more engine control interfaces including sensed temperatures and other values that may impact timing decisions for alternating starter use for each engine. An engine control interface that receives the parameters can determine present conditions with respect to an operating envelope of a power source (e.g., compressed air or electric) and the starting system of each engine based on predetermined performance constraints, engine drag, and/or parasitic factors. An alternating pattern of starter system acceleration/deceleration can be commanded to perform dry motoring in multiple engines at the same time in a staggered manner based on determining that each of the starting systems will receive a sufficient supply of power (e.g., continue rotating and stay below the resonance speed) using the alternating pattern based on the present conditions. The engine control interface can continue monitoring performance of the engines during dry motoring and adjust the command timing as needed, for instance, to account for performance variations over time.

Referring now to FIG. 1 , a schematic illustration of an aircraft 5 is depicted with a pair of engine systems 100A, 100B. Engine systems 100A, 100B include gas turbine engines 10A, 10B and engine starting systems 101A, 101B respectively. Engine systems 100A, 100B also include FADECs 102A, 102B to control gas turbine engines 10A, 10B and starting systems 101A, 101B. FADECs 102A, 102B may generally be referred to as controllers. FADECs 102A, 102B can communicate with respective engine control interfaces 105A, 105B using a digital communication bus 106. The engine control interfaces 105A, 105B can buffer engine system communication from aircraft level communication. Although depicted separately in FIG. 1 , in some embodiments the engine control interfaces 105A, 105B are integrated with the FADECs 102A, 102B. The engine control interfaces 105A, 105B may also be referred to as controllers when configured to make mode selection determinations to perform single engine or multi-engine dry motoring for the aircraft 5.

In an embodiment, the FADECs 102A, 102B and engine control interfaces 105A, 105B may each include memory to store instructions that are executed by one or more processors on one or more channels. The executable instructions may be stored or organized in any manner and at any level of abstraction, such as in connection with a controlling and/or monitoring operation of the gas turbine engines 10A, 10B of FIG. 1 . The one or more processors can be any type of central processing unit (CPU), including a general purpose processor, a digital signal processor (DSP), a microcontroller, an application specific integrated circuit (ASIC), a field programmable gate array (FPGA), or the like. Also, in embodiments, the memory may include random access memory (RAM), read only memory (ROM), or other electronic, optical, magnetic, or any other computer readable medium onto which is stored data and control algorithms in a non-transitory form.

In the example of FIG. 1 , an auxiliary power unit (APU) 113 and compressor 115 provide a compressed air source 114 to drive air turbine starters 120A, 120B of engine starting systems 101A, 101B. The APU 113 can also produce electric power. Compressed air from the compressed air source 114 is routed through ducts 117 and air starter valves 116A, 116B to the air turbine starters 120A, 120B. Various shutoff valves can also be included in ducts 117, such as a main shutoff valve 119 and engine shutoff valves 121A, 121B. One or more bleed valves 123 can be used to release compressed air from the ducts 117.

In some cases, dry motoring can be performed simultaneously for engine systems 100A, 100B, where compressed air from the compressed air source 114 is provided to both air turbine starters 120A, 120B in an alternating pattern. The FADECs 102A, 102B and/or the engine control interfaces 105A, 105B can be configured with control laws to oscillate a motoring speed in an alternating pattern between each of the gas turbine engines 10A, 10B and maintain the motoring speed below a threshold level (i.e., the critical rotor speed) for the engine system 100A, 100B while performing dry motoring based on compressed air source 114. In embodiments, FADECs 102A, 102B can observe various engine parameters and starting system parameters to actively control dry motoring and prevent fault conditions from damaging the gas turbine engines 10A, 10B. For example, FADECs 102A, 102B can observe engine speeds (N2) of gas turbine engines 10A, 10B and may receive starter system parameters such as starter speeds (NS) and/or starter air pressures (SAP). In embodiments, FADECs 102A, 102B can adjust starter air valves 116A, 116B based on commands received from the engine control interfaces 105A, 105B to prevent both starter air valves 116A, 116B being simultaneously fully open for an extended period of time and balance loading on the compressed air source 114.

To further enhance control aspects, the FADECs 102A, 102B can provide either or both of the engine control interfaces 105A, 105B with engine data including parameters that directly or indirectly modify an aspect of the compressed air received at the starter air valves 116A, 116B. Engine data can be sent on the digital communication bus 106 to either or both of the engine control interfaces 105A, 105B to make relative timing control determinations. Engine data can also or alternatively be exchanged on an alternate link (e.g., cross engine bus 106A of FIG. 2 ) between FADECs 102A, 102B for relative timing control. Engine data may include fault information, such as a detected failure of the starter air valves 116A, 116B and/or the air turbine starters 120A, 120B. Present condition information and/or commands included in the engine data can allow the engine control interfaces 105A, 105B to track and/or predict events that will impact available compressed air for dry motoring at each of the engine starting systems 101A, 101B. For example, at least one temperature of gas turbine engines 10A, 10B, such as a measured core engine temperature or an oil temperature, can be used to determine current conditions and select timing parameters for alternating acceleration/deceleration patterns between two or more engines 10A, 10B at the same time. Additional performance parameters, such as an ambient air temperature and/or an ambient pressure, can also be used to determine relative control timing between two or more engines 10A, 10B. For instance, ambient temperature can be used for temperature comparison/normalization and ambient pressure can be used to adjust for altitude effects. When sufficient margin is present, a greater amount of overlap may be commanded where both starter air valves 116A, 116B are simultaneously open. Where reduced compressed air source capacity is available, only one of the starter air valves 116A, 116B may be open at a time.

Although FIG. 1 depicts one example configuration, it will be understood that embodiments as described herein can cover a wide range of configurations, such as a four engine system. Further, the compressed air source 114 can include multiple sources other than APU 113 and compressor 115, such as a ground cart or cross engine bleed air. The compressed air source 114 is an example of a power source for starting systems 101A, 101B. In alternate embodiments, a power source can be electric power when electric starters are used in place of air turbine starters 120A, 120B.

Turning now to FIG. 2 , a schematic of engine systems 100A, 100B and engine starting systems 101A, 101B for the gas turbine engines 10A, 10B of FIG. 1 are depicted according to an embodiment. In the example of FIG. 2 , the digital communication bus 106 can include an aircraft, engine, and/or test stand communication bus to interface with FADECs 102A, 102B, engine control interfaces 105A, 105B, aircraft controls, e.g., a cockpit, various onboard computer systems, and/or a test stand (not depicted). In some embodiments, a cross engine bus 106A provides a link between FADECs 102A, 102B as a lower latency communication path between engine systems 100A, 100B, for instance, by avoiding shared communication scheduling conflicts with other aircraft level systems. Either or both channels of FADECs 102A, 102B can alternate on and off commands to respective electromechanical devices 110A, 110B coupled to starter air valves 116A, 116B to achieve a partially open position of the starter air valves 116A, 116B to control a flow of compressed air from compressed air source 114 (e.g., APU 113 and compressor 115 of FIG. 1 ) as a starter air flow to air turbine starters 120A, 120B during dry motoring. The air turbine starters 120A, 120B output torque to drive rotation of respective gas turbine engine shafts 50A, 50B of starting spools of the gas turbine engines 10A, 10B.

The FADECs 102A, 102B can monitor engine speed (N2), starter speed (NS), starter air pressure (SAP), and/or other engine parameters to determine an engine operating state and control the starter air valves 116A, 116B. Thus, the FADECs 102A, 102B can each establish a control loop with respect to a motoring speed (N2 and/or NS) and/or starter air pressure to adjust positioning of the starter air valves 116A, 116B. The FADECs 102A, 102B can also transmit engine data on digital communication bus 106 to engine control interfaces 105A, 105B, including present conditions and commands of each engine system 100A, 100B that may impact characteristics of the compressed air available at the starter air valves 116A, 116B. The engine control interfaces 105A, 105B can supply the FADECs 102A, 102B with coordination commands to adjust relative timing between the FADECs 102A, 102B such that loading on the compressed air source 114 is balanced by staggering the timing of open/close commands between each of the starter air valves 116A, 116B.

In some embodiments, the starter air valves 116A, 116B are discrete valves designed as on/off valves that are typically commanded to either fully opened or fully closed. However, there is a time lag to achieve the fully open position and the fully closed position. By selectively alternating an on-command time with an off-command time through the electromechanical devices 110A, 110B, intermediate positioning states (i.e., partially opened/closed) can be achieved. The FADECs 102A, 102B can modulate the on and off commands (e.g., as a duty cycle using pulse width modulation) to the electromechanical devices 110A, 110B to further open the starter air valves 116A, 116B and increase a rotational speed of the gas turbine engine shafts 50A, 50B. In an embodiment, the electromechanical devices 110A, 110B have a cycle time defined between an off-command to an on-command to the off-command that is at most half of a movement time for the starter air valves 116A, 116B to transition from fully closed to fully open. Pneumatic lines or mechanical linkage (not depicted) can be used to drive the starter air valves 116A, 116B between the open position and the closed position. The electromechanical devices 110A, 110B can each be a solenoid that positions the starter air valves 116A, 116B based on intermittently supplied electric power as commanded by the FADECs 102A, 102B. In an alternate embodiment, the electromechanical devices 110A, 110B are electric valves controlling muscle air to adjust the position of the starter air valves 116A, 116B as commanded by the FADECs 102A, 102B.

In an alternate embodiment, rather than using electromechanical devices 110A, 110B to achieve a partially open position of the starter air valves 116A, 116B, the starter air valves 116A, 116B can be variable position valves that are dynamically adjustable to selected valve angles by the FADECs 102A, 102B. When implemented as variable position valves, the starter air valves 116A, 116B can be continuous/infinitely adjustable and hold a commanded valve angle, which may be expressed in terms of a percentage open/closed and/or an angular value (e.g., degrees or radians). Performance parameters of the starter air valves 116A, 116B can be selected to meet dynamic response requirements. For example, in some embodiments, the starter air valves 116A, 116B each have a response rate of 0% to 100% open in less than 40 seconds. In other embodiments, the starter air valves 116A, 116B each have a response rate of 0% to 100% open in less than 30 seconds. In further embodiments, the starter air valves 116A, 116B each have a response rate of 0% to 100% open in less than 20 seconds.

In some embodiments, the FADECs 102A, 102B can each monitor a valve angle of the starter air valves 116A, 116B when valve angle feedback is available. The FADECs 102A, 102B can establish an outer control loop with respect to motoring speed and an inner control loop with respect to the valve angle of the starter air valves 116A, 116B. Valve angle feedback and/or valve commands can be included in the cross engine data exchanged between the FADECs 102A, 102B and may be sent to the engine control interfaces 105A, 105B. In some embodiments, the engine control interfaces 105A, 105B establish a further outer control loop that adjusts timing of control actions of the FADECs 102A, 102B relative to each other.

As a further alternative, FIG. 10 depicts a schematic of engine systems 400A, 400B and engine starting systems 401A, 401B for the gas turbine engines 10A, 10B of FIG. 1 according to an embodiment. In the example of FIG. 10 , either or both channels of FADECs 102A, 102B can alternate on and off commands to respective starter switches 416A, 416B to control a flow of electric current from electric power source 414 (e.g., electricity generated by APU 113 of FIG. 1 , a stored energy source, a cross engine power supply, ground power, etc.) as a starter current to electric starters 420A, 420B during dry motoring. The electric starters 420A, 420B output torque to drive rotation of respective gas turbine engine shafts 50A, 50B of starting spools of the gas turbine engines 10A, 10B.

The FADECs 102A, 102B can monitor engine speed (N2), starter speed (NS), current, and/or other engine parameters to determine an engine operating state and control the starter switches 416A, 416B. Thus, the FADECs 102A, 102B can each establish a control loop with respect to a motoring speed (N2 and/or NS) and/or current to adjust the state of the starter switches 416A, 416B. The FADECs 102A, 102B can also transmit engine data on digital communication bus 106 to engine control interfaces 105A, 105B, including present conditions and commands of each engine system 100A, 100B that may impact characteristics of the electric current available at the starter switches 416A, 416B. The engine control interfaces 105A, 105B can supply the FADECs 102A, 102B with coordination commands to adjust relative timing between the FADECs 102A, 102B such that loading on the electric source 414 is balanced by staggering the timing of on/off commands between each of the starter switches 416A, 416B.

FIGS. 3 and 4 depict two example engine configurations of the gas turbine engines 10A, 10B of FIG. 1 . FIG. 3 is an example of a straddle-mounted spool 32A as a starting spool configuration. This configuration places two bearing compartments 37A and 39A (which may include a ball bearing and a roller bearing respectively), outside of the plane of most of the compressor disks of high pressure compressor 52A and at outside at least one of the turbine disks of high pressure turbine 54A. In contrast with a straddle-mounted spool arrangement, other embodiments may be implemented using an over-hung mounted spool 32B as depicted in FIG. 4 as a starting spool configuration. In over-hung mounted spool 32B, a bearing compartment 37B is located forward of the first turbine disk of high pressure turbine 54B such that the high pressure turbine 54B is overhung, and it is physically located aft of its main supporting structure. The use of straddle-mounted spools has advantages and disadvantages in the design of a gas turbine, but one characteristic of the straddle-mounted design is that the span between the bearing compartments 37A and 39A is long, making the amplitude of the high spot of a bowed rotor greater and the resonance speed that cannot be transited prior to temperature homogenization is lower. For any thrust rating, the straddle mounted arrangement, such as straddle-mounted spool 32A, gives Lsupport/Dhpt values that are higher, and the overhung mounted arrangement, such as overhung spool 32B, can be as much as 60% of the straddle-mounted Lsupport/Dhpt. Lsupport is the distance between bearings (e.g., between bearing compartments 37A and 39A or between bearing compartments 37B and 39B), and Dhpt is the diameter of the last blade of the high pressure turbine (e.g., high pressure turbine 54A or high pressure turbine 54B). As one example, a straddle-mounted engine starting spool, such as straddle-mounted spool 32A, with a roller bearing at bearing compartment 39A located aft of the high pressure turbine 54A may be more vulnerable to bowed rotor problems since the Lsupport/Dhpt ranges from 1.9 to 5.6.

FIGS. 3 and 4 also illustrate a starter as an air turbine starter 120 (e.g., air turbine starter 120A or 120B of FIGS. 1 and 2 ) or electric starter 420 (e.g., electric starter 420A or 420B of FIG. 10 ) interfacing through gearbox 124 via a tower shaft 55 with the straddle-mounted spool 32A proximate high compressor 52A and interfacing via tower shaft 55 with the overhung mounted spool 32B proximate high compressor 52B as part of a starting system. The straddle-mounted spool 32A and the over-hung mounted spool 32B are both examples of a starter spool having a gas turbine engine shaft 50 driven by the starter 120, 420, such as gas turbine engine shafts 50A, 50B driven by air turbine starters 120A, 120B of FIG. 2 or electric starters 420A, 420B of FIG. 10 .

FIG. 5 is a block diagram of a system 200 for bowed rotor start mitigation that may control either of the starter air valves 116A, 116B of FIGS. 1 and 2 or the starter switches 416A, 416B of FIG. 10 via control signals 210 in accordance with an embodiment. The system 200 may also be referred to as a bowed rotor start mitigation system. In the example of FIG. 5 , the system 200 includes an onboard model 202 operable to produce a compressor exit temperature T₃ and a compressor inlet flow W₂₅ of one of the gas turbine engines 10A, 10B of FIG. 1 for use by a core temperature model 204. The onboard model 202 is configured to synthesize or predict major temperatures and pressures throughout one of the gas turbine engines 10A, 10B of FIG. 1 beyond those sensed by sensors positioned about the gas turbine engines 10A, 10B. The onboard model 202 and core temperature model 204 are examples of a first thermal model and a second thermal model that may be separately implemented or combined as part of a controller 102 (e.g., FADECs 102A, 102B of FIG. 1 ).

Engine parameter synthesis is performed by the onboard model 202, and the engine parameter synthesis may be performed using the technologies described in U.S. Patent Publication No. 2011/0077783, the entire contents of which are incorporated herein by reference thereto. Of the many parameters synthesized by onboard model 202 at least two are outputted to the core temperature model 204, T₃, which is the compressor exit gas temperature of each gas turbine engine 10A, 10B and W₂₅, which is the air flow through the compressor. Each of these values are synthesized by onboard model 202 and inputted into the core temperature model 204 that synthesizes or provides a heat state (T_(core)) of each gas turbine engine 10A, 10B. T_(core) can be determined by a first order lag or function of T₃ and a numerical value X (e.g., f(T₃, X)), wherein X is a value determined from a lookup table stored in memory of controller 102. Accordingly, X is dependent upon the synthesized value of W₂₅. In other words, W₂₅ when compared to a lookup table of the core temperature model 204 will determine a value X to be used in determining the heat state or T_(core) of each gas turbine engine 10A, 10B. In one embodiment, the higher the value of W₂₅ or the higher the flow rate through the compressor the lower the value of X.

The heat state of each engine 10A, 10B during use or T_(core) is determined or synthesized by the core temperature model 204 as each engine 10A, 10B is being run. In addition, T₃ and W₂₅ are determined (e.g., measured) or synthesized by the onboard model 202 and/or the controller 102 as each engine 10A, 10B is being operated.

At engine shutdown, the current or most recently determined heat state of the engine or T_(core shutdown) of an engine 10A, 10B is recorded into data storage unit (DSU) 104, and the time of the engine shutdown t_(shutdown) is recorded into the DSU 104. The DSU 104 retains data between shutdowns using non-volatile memory. Each engine 10A, 10B may have a separate DSU 104. Time values and other parameters may be received on digital communication bus 106 and/or cross engine bus 106A. As long as electrical power is present for the controller 102 and DSU 104, additional values of temperature data may be monitored for comparison with modeled temperature data to validate one or more temperature models (e.g., onboard model 202 and/or core temperature model 204) of each gas turbine engine 10A, 10B.

During an engine start sequence or restart sequence, a bowed rotor start risk model 206 (also referred to as risk model 206) of the controller 102 is provided with the data stored in the DSU 104, namely T_(core shutdown) and the time of the engine shutdown t_(shutdown). In addition, the bowed rotor start risk model 206 is also provided with the time of engine start t_(start) and the ambient temperature of the air provided to the inlet of each engine 10A, 10B T_(inlet) or T₂. T₂ is a sensed value as opposed to the synthesized value of T₃ in some embodiments. In some embodiments, an oil temperature (T_(oil)) is a sensed value that can be used to determine a current temperature in combination with T_(core) and/or T₂. For instance, once oil stops circulating at shutdown, T_(oil) can provide a localized temperature reading indicative of a bearing compartment temperature from which temperatures at various engine locations can be derived.

The bowed rotor start risk model 206 maps core temperature model data with time data and ambient temperature data to establish a motoring time t_(motoring) as an estimated period of motoring to mitigate a bowed rotor of each gas turbine engine 10A, 10B. The motoring time t_(motoring) is indicative of a bowed rotor risk parameter computed by the bowed rotor start risk model 206. For example, a higher risk of a bowed rotor may result in a longer duration of dry motoring to reduce a temperature gradient prior to starting each gas turbine engine 10A, 10B of FIG. 1 . In one embodiment, an engine start sequence may automatically include a modified start sequence; however, the duration of the modified start sequence prior to a normal start sequence will vary based upon the time period t_(motoring) that is calculated by the bowed rotor start risk model 206. The motoring time t_(motoring) for predetermined target speed N_(target) of each engine 10A, 10B is calculated as a function of T_(core shutdown), t_(shutdown), t_(start), T₂ and/or T_(oil), (e.g., f(T_(core shutdown), t_(shutdown), t_(start), T₂ and/or T_(oil)), while a target speed N_(target) is a predetermined speed that can be fixed or vary within a predetermined speed range of N_(targetMin) to N_(targetMax). In other words, the target speed N_(target) may be the same regardless of the calculated time period t_(motoring) or may vary within the predetermined speed range of N_(targetMin) to N_(targetMax). The target speed N_(target) may also be referred to as a dry motoring mode speed. In some embodiments, the target speed N_(target) defines a speed threshold that may be set statically or dynamically adjusted based on coordination commands 205 from engine control interfaces 105A, 105B to determine when to command a corresponding starter air valve 116A, 116B or a corresponding starter switch 416A, 416B to close/open, for example.

Based upon these values (T_(core shutdown), t_(shutdown), t_(start), T₂ and/or T_(oil)) the motoring time t_(motoring) for the modified start sequence of each engine 10A, 10B is determined by the bowed rotor start risk model 206. Based upon the calculated time period t_(motoring) which is calculated as a time to run each engine 10A, 10B up to a predetermined target speed N_(target) in order to clear a “bowed condition”. In accordance with an embodiment of the disclosure, the controller 102 can run through a modified start sequence upon a start command given to each engine 10A, 10B by an operator of the engines 10A, 10B, such as a pilot of an airplane the engines 10A, 10B are used with. It is understood that the motoring time t_(motoring) of the modified start sequence may be in a range of 0 seconds to minutes, which depends on the values of T_(core shutdown), t_(shutdown), t_(start), T₂ and/or T_(oil).

In an alternate embodiment, the modified start sequence may only be run when the bowed rotor start risk model 206 has determined that the motoring time t_(motoring) is greater than zero seconds upon receipt of a start command given to each engine 10A, 10B. In this embodiment and if the bowed rotor start risk model 206 has determined that t_(motoring) is not greater than zero seconds, a normal start sequence will be initiated upon receipt of a start command to each engine 10A, 10B.

Accordingly and during an engine command start, the bowed rotor start risk model 206 of the system 200 may be referenced wherein the bowed rotor start risk model 206 correlates the elapsed time since the last engine shutdown time and the shutdown heat state of each engine 10A, 10B as well as the current start time t_(start) and the inlet air temperature T₂ in order to determine the duration of the modified start sequence wherein motoring of each engine 10A, 10B up to a reduced speed N_(target) without fuel and ignition is required. As used herein, motoring of each engine 10A, 10B in a modified start sequence refers to the turning of a starting spool by air turbine starter 120A, 120B or electric starter 420A, 420B up to a reduced speed N_(target) without introduction of fuel and an ignition source in order to cool the engine 10A, 10B to a point wherein a normal start sequence can be implemented without starting the engine 10A, 10B in a bowed rotor state. In other words, cool or ambient air is drawn into the engine 10A, 10B while motoring the engine 10A, 10B at a reduced speed in order to clear the “bowed rotor” condition, which is referred to as a dry motoring mode.

The bowed rotor start risk model 206 can output the motoring time t_(motoring) to a motoring controller 208. The motoring controller 208 uses a dynamic control calculation in order to determine a required valve position of the starter air valve 116A, 116B used to supply an air supply or compressed air source 114 to the engine 10A, 10B in order to limit the motoring speed of the engine 10A, 10B up to the target speed N_(target) due to the position of the starter air valve 116A, 116B. The required valve position of the starter air valve 116A, 116B can be determined based upon an air supply pressure as well as other factors including but not limited to ambient air temperature, parasitic drag on the engine 10A, 10B from a variety of engine driven components such as electric generators and hydraulic pumps, and other variables such that the motoring controller 208 closes the loop for engine motoring for the required amount of time based on the output of the bowed rotor start risk model 206. Similar computations can be performed for switch timing of the starter switches 416A, 416B, which may be limited by a maximum current limit of the electric power source 414 of FIG. 10 . Engine data 209 can also be transmitted on digital communication bus 106 to the engine control interfaces 105A, 105B including present conditions, commands, and/or scheduled adjustments of the starting system 101A, 101B to assist the engine control interfaces 105A, 105B in making timing determinations as coordination commands 205 to alternate compressed air use by the starting systems 101A, 101B or electric current use by starting systems 401A, 401B of FIG. 10 during dry motoring. For instance, engine data 209 can include a measured core engine temperature, an oil temperature, an ambient air temperature, an ambient pressure, a starting spool speed, and the like. Engine data 209 can also include a number of control parameters such as a baseline dry motoring profile and/or adjustments to the baseline dry motoring profile (e.g., timing adjustments). The coordination commands 205 can be used to extend or delay dry motoring on a per engine basis and/or change a motoring target speed threshold. Changes may include extending the motoring time requirements predicted by the bowed rotor start risk model 206 and/or modifying a selection or applying a rescaling to a baseline dry motoring profile determined by the bowed rotor start risk model 206. In alternate embodiments, the bowed rotor start risk model 206 is omitted and motoring timing and/or other motoring constraints are received in coordination commands 205.

In one embodiment, the dynamic control of the valve position (e.g., open state of the valve (e.g., fully open, ½ open, ¼ open, etc.) in order to limit the motoring speed of the engine 10A, 10B) is controlled via duty cycle control (on/off timing using pulse width modulation) of electromechanical device 110A, 110B for starter air valves 116A, 116B. When variable position starter air valves are used as the starter air valves 116A, 116B, a valve angle 207 can be provided to motoring control 208 based on valve angle feedback. A rotor speed N2 can be provided to the motoring controller 208 and a mitigation monitor 214, where motoring controller 208 and a mitigation monitor 214 may be part of controller 102.

The risk model 206 can determine a bowed rotor risk parameter that is based on the heat stored (T_(core)) using a mapping function or lookup table. When not implemented as a fixed rotor speed, the bowed rotor risk parameter can have an associated dry motoring profile defining a target rotor speed profile over an anticipated amount of time for the motoring controller 208 to send control signals 210, such as valve control signals for controlling starter air valves 116A, 116B of FIG. 3 or starter switches 416A, 416B of FIG. 10 .

The bowed rotor risk parameter may be quantified according to a profile curve selected from a family of curves that align with observed aircraft/engine conditions that impact turbine bore temperature and the resulting bowed rotor risk. In some embodiments, an anticipated amount of dry motoring time can be used to determine a target rotor speed profile in a dry motoring profile for the currently observed conditions. As one example, one or more baseline characteristic curves for the target rotor speed profile can be defined in tables or according to functions that may be rescaled to align with the observed conditions.

In summary with reference to FIG. 5 , as one example of an aircraft 5 that includes systems as described herein, onboard model 202 and core temperature model 204 may run on controller 102 of the aircraft 5 to track heat stored (T_(core)) in the turbine at the time of engine shutdown. Modeling of potential heat stored in the system may be performed as a turbine disk metal temperature model in the core temperature model 204. When the aircraft lands, engines typically operate at idle for a cool down period of time, e.g., while taxiing to a final destination. When an engine shutdown is detected, model state data can be logged by the DSU 104 prior to depowering. When the controller 102 powers on at a later time and model state data can be retrieved from the DSU 104, and the bowed rotor start risk model 206 can be updated to account for the elapsed time. When an engine start is requested, a bowed rotor risk can be assessed with respect to the bowed rotor start risk model 206. Extended dry motoring can be performed during an engine start process until the bow risk has sufficiently diminished. The state of or changes to the coordination commands 205 can start/stop dry motoring and/or result in adjustments to motoring time and/or a dry motoring profile used to drive control signals 210 to reduce the amount of time that both air turbine starters 120A, 120B (or electric starters 420A, 420B) are simultaneously being fully driven (e.g., establish a time delay/phase shift between acceleration/deceleration profiles between engines).

In reference to FIG. 5 , the mitigation monitor 214 can operate in response to receiving a complete indicator 212 to run a verification of the bowed rotor mitigation. The mitigation monitor 214 can provide mitigation results 216 to the motoring controller 208 and may provide result metrics 218 to other systems, such a maintenance request or indicator. The mitigation monitor 214 may also run while dry motoring is active to determine whether adjustments to the dry motoring profile are needed. If the mitigation monitor 214 determines that a bowed rotor condition still exists, the motoring controller 208 may restart dry motoring, or a maintenance request or indicator can be triggered along with providing result metrics 218 for further analysis. Metrics of attempted bowed rotor mitigation can be recorded in the DSU 104 based on determining that the attempted bowed rotor mitigation was unsuccessful or incomplete. Mitigation results 216 and/or result metrics 218 may also be included in the engine data 209 sent to engine control interfaces 105A, 105B of FIG. 1 .

FIG. 6 is a block diagram of multi-engine coordination command logic 250 for alternating starter use during multi-engine motoring in accordance with an embodiment. Although inputs from only two engines are depicted in the example of FIG. 6 , it will be understood that the design can scale to any number of engines (e.g., a four engine aircraft). The multi-engine coordination command logic 250 may be part of either or both of the engine control interfaces 105A, 105B of FIG. 1 .

In the example of FIG. 6 , the multi-engine coordination command logic 250 includes a coordination controller 252 and a feedback monitor 254. The coordination controller 252 can receive engine data 209 from FADECs 102A, 102B including a first engine temperature value 256A and a second engine temperature value 256B. Various parameters can be received or otherwise determined by the engine control interfaces 105A, 105B, such as starter parameters 258, power source parameters 260, engine drag 262, parasitic factors 264, and flight regime 266. The starter parameters 258 can include performance limitations of the air turbine starters 120A, 120B or electric starters 420A, 420B. The power source parameters 260 can include performance limitations of the compressed air source 114, such as output limits as a function of pressure and/or temperature, or electric power source 414 with electric current limits. The engine drag 262 may be determined as a function of oil temperature and/or ambient conditions for each of the engines 10A, 10B. The parasitic factors 264 can be defined as correlations with respect to one or more of an ambient temperature, engine core temperature, and/or oil temperature. The flight regime 266 can be used to determine when dry motoring should be enabled or inhibited. For instance, dry motoring can be inhibited when the aircraft 5 of FIG. 1 is not on the ground. The various parameters available to the coordination controller 252 enable selection of a dry motoring timing sequence that corresponds with the current conditions to determine when the system is collectively capable of performing alternating starter air use during multi-engine motoring such that multiple gas turbine engines 10A, 10B can alternate reaching a targeted motoring speed or track a desired dry motoring profile at alternating times. Results of the determination of the coordination controller 252 are output as coordination commands 205, for instance, first engine motoring coordination commands 205A to FADEC 102A and second engine motoring coordination commands 205B to FADEC 102B.

The feedback monitor 254 can also receive engine data 209, which may include a first engine speed (e.g., N2 of engine 10A) 270A, a first starter status 272A (e.g., open/close command/status of starter air valve 116A or starter switch 416A), a second engine speed (e.g., N2 of engine 10B) 270B, and a second starter status 272B (e.g., open/close command/status of starter air valve 116B or starter switch 416B). When dry motoring is active in engines 10A, 10B, the feedback monitor 254 can monitor the speed (e.g., first and second engine speed 270A, 270B) of each of the engines 10A, 10B and/or the first and second starter status 272A, 272B to determine a need to adjust open/close valve timing for the starter air valve 116A, 116B and/or other valves or electric switches 416A, 416B. The feedback monitor 254 can send control parameter feedback 274 to the coordination controller 252, for instance, indicating whether the first or second engine speed 270A, 270B has reached one or more thresholds 276 such that a valve/switch open/close command should be issued. In some embodiments, the thresholds 276 include a first and second speed threshold that can be dynamically adjusted over time. For example, speed values used as the thresholds 276 can change over time as dry motoring continues according to a dry motoring profile or in response to a change in power supply conditions (e.g., output pressure reduction from compressed air source 114 or output current reduction from electric power source 414). In some embodiments, the engine data 209 includes a currently targeted speed for either or both engines 10A, 10B when dynamic adjustments to the thresholds 276 are permitted. In other embodiments, the FADECs 102A, 102B each self-report to the feedback monitor 254 an indication of whether each respective engine 10A, 10B is achieving respective targeted speeds/performance during dry motoring. Upon receiving the control parameter feedback 274, the coordination controller 252 can modify the first and/or second engine motoring coordination commands 205A, 205B to increase a starting spool speed in one of the engines 10A, 10B while decreasing the starting spool speed in the other engine 10B, 10A.

FIG. 7 depicts relative starter air valve timing for multiple starter air valves in accordance with an embodiment. For example, starter air valve 116A may transition from closed to open in response to valve command 280A, while starter air valve 116B remains closed. When valve command 280A commands starter air valve 116A to at least partially close, starter air valve 280B can be commanded open. In some instances, there can be partial overlaps in time 282 where starter air valve 116A is commanded open before starter air valve 116B is commanded closed. However, the general pattern as depicted in the example of FIG. 7 is alternating opening/closing of each starter air valve 116A, 116B as dry motoring is performed. Similarly, electric switches 416A, 416B can transition in a staggered pattern but with an opposite phase as depicted in FIG. 7 .

FIG. 8 depicts an example of engine speed oscillations for multiple engines in accordance with an embodiment. When control is performed with respect to engine speed (e.g., N2), starting spool speed 290A of gas turbine engine 10A can increase until a first speed threshold 292A is reached. Reaching the first speed threshold 292A may correspond in a transition of valve command 280A of FIG. 7 from open to at least partially closed such that starting spool speed 290A transitions from accelerating to decelerating. Similarly, when valve command 280B of FIG. 7 transitions from closed to open, starting spool speed 290B of gas turbine engine 10B can increase until a second speed threshold 292B is reached. Reaching the second speed threshold 292B may correspond in a transition of valve command 280B of FIG. 7 from open to at least partially closed such that starting spool speed 290B transitions from accelerating to decelerating. The process of alternating acceleration and deceleration of the starting spool speed 290A, 290B in an opposite phase relationship can continue until a motoring termination condition is met (e.g., a motoring time limit is reached, a failure occurs, a temperature limit is reached, an override command is received, etc.). There can be some overlap 294 where both starter air valves 116A, 116B are partially opened. A similar pattern can be achieved by staggering the opening and closing (on/off state) of starter switches 416A, 416B in the embodiment of FIG. 10 .

In some embodiments, the first and second speed thresholds 292A and 292B have different values set based on engine system specific parameters, e.g., time in service, manufacturing variations, and the like. In other embodiments, the first and second speed thresholds 292A and 292B are the same. The first and second speed thresholds 292A and 292B can be dynamically modified to move closer to a resonance speed 296 without exceeding the resonance speed 296 as part of an extended dry motoring profile. The first and second speed thresholds 292A and 292B may be set or modified based on ambient conditions and current performance of the power source, such as compressed air source 114 or electric power source 414, for instance. As one example, compressed air or electric current provided by a ground cart may have different characteristics than when provided by APU 113 of FIG. 1 .

FIG. 9 is a flow chart illustrating a method 300 for alternating starter use during multi-engine motoring in accordance with an embodiment. The method 300 of FIG. 9 is described in reference to FIGS. 1-8 and may be performed with an alternate order and include additional steps. At block 302, a first controller (e.g., FADEC 102A) commands a first starter air valve 116A of a first gas turbine engine 10A to open while a second starter air valve 116B of a second gas turbine engine 10B is closed during motoring. At block 304, the first controller (e.g., FADEC 102A) commands the first starter air valve 116A to at least partially close based on a starting spool speed 290A of the first gas turbine engine 10A reaching a first speed threshold 292A. At block 306, a second controller (e.g., FADEC 102B) commands the second starter air valve 116B to open after the first starter air valve 116A is at least partially closed. At block 308, the second controller (e.g., FADEC 102B) commands the second starter air valve 116B to at least partially close based on the starting spool speed 290B of the second gas turbine engine 10B reaching a second speed threshold 292B. At block 310, alternating opening and closing of the first starter air valve 116A and the second starter air valve 116B is repeated until a motoring termination condition is met. A similar alternating pattern can be performed with respect to the engine starting systems 401A, 401B of FIG. 10 . The alternating opening and closing can include partially opening and partially closing the first and second air valves 116A, 116B.

At least one engine control interface 105A, 105B can coordinate command sequencing of the first controller and the second controller to perform intermittent acceleration and deceleration of the starting spool of the first gas turbine engine 10A and the second gas turbine engine 10B in an alternating sequence during motoring. Timing of coordinated control between the first controller and the second controller can be determined based on a plurality of performance parameters that are based on one or more of: an ambient condition, performance limitations of a compressed air source 114 and first and second air turbine starters 120A, 120B driven by the compressed air source 114 in response to the first and second starter air valves 116A, 116B, engine drag, and parasitic factors. The compressed air source 114 can be an auxiliary power unit 113 of the aircraft 5, a ground cart, or a cross engine bleed. The performance parameters can be determined based on one or more of: an ambient air temperature (e.g., T_(amb) or T₂), an ambient pressure (e.g., P_(amb)), and an oil temperature (e.g., T_(oil)). The first speed threshold 292A and the second speed threshold 292B are dynamically configurable and below a resonance speed 296 of the starting spool speed 290A, 290B of the first gas turbine engine 10A and the second gas turbine engine 10B.

In alternate embodiments, various control parameters can be used to achieve an alternating/phase shifted dry motoring timing between gas turbine engines 10A, 10B. For example, control can be coordinated based on one or more of: an engine speed, a starter speed, a starter valve position, a starter air pressure, a starter system electric current, a rate of speed change, a rate of starter valve position change, a rate of starter air pressure change, and a rate starter system electric current change. In some embodiments, at least one engine control interface 105A, 105B coordinates control of the first starter air valve 116A through FADEC 102A and the second starter air valve 116B through FADEC 102B to alternate use of the compressed air between the first air turbine starter 120A and the second air turbine starter 120B while maintaining a starting spool speed 290A, 290B of the first gas turbine engine 10A and the second gas turbine engine 10B below a resonance speed 296 during motoring. Similarly, at least one engine control interface 105A, 105B can coordinate control of the first starter switch 416A through FADEC 102A and the second starter switch 416B through FADEC 102B to alternate use of the electric current between the first electric starter 420A and the second electric starter 420B while maintaining a starting spool speed 290A, 290B of the first gas turbine engine 10A and the second gas turbine engine 10B below a resonance speed 296 during motoring. Control may also be coordinated between FADECs 102A, 102B and/or engine control interfaces 105A, 105B by commanding one or more adjustment to a baseline dry motoring profile to shift timing of the use of the compressed air on an engine basis.

Accordingly and as mentioned above, it is desirable to detect, prevent and/or clear a “bowed rotor” condition in a gas turbine engine that may occur after the engine has been shut down. As described herein and in one non-limiting embodiment, the FADECs 102A, 102B (e.g., controller 102) and/or engine control interfaces 105A, 105B may be programmed to automatically take the necessary measures in order to provide for a modified start sequence without pilot intervention other than the initial start request. In an exemplary embodiment, the FADECs 102A, 102B, DSU 104 and/or engine control interfaces 105A, 105B each comprises a microprocessor, microcontroller or other equivalent processing device capable of executing commands of computer readable data or program for executing a control algorithm and/or algorithms that control the start sequence of the gas turbine engine. In order to perform the prescribed functions and desired processing, as well as the computations therefore (e.g., the execution of Fourier analysis algorithm(s), the control processes prescribed herein, and the like), the FADECs 102A, 102B, DSU 104 and/or engine control interfaces 105A, 105B may include, but not be limited to, a processor(s), computer(s), memory, storage, register(s), timing, interrupt(s), communication interfaces, and input/output signal interfaces, as well as combinations comprising at least one of the foregoing. For example, the FADECs 102A, 102B, DSU 104 and/or engine control interfaces 105A, 105B may include input signal filtering to enable accurate sampling and conversion or acquisitions of such signals from communications interfaces. As described above, exemplary embodiments of the disclosure can be implemented through computer-implemented processes and apparatuses for practicing those processes.

While the present disclosure has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the present disclosure is not limited to such disclosed embodiments. Rather, the present disclosure can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the present disclosure. Additionally, while various embodiments of the present disclosure have been described, it is to be understood that aspects of the present disclosure may include only some of the described embodiments. Accordingly, the present disclosure is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims. 

What is claimed is:
 1. A system for alternating starter use during multi-engine motoring in an aircraft, the system comprising: a first engine starting system of a first engine; and a first controller in communication with a second controller that controls a second engine starting system of a second engine, the first controller being configured to intermittently direct power to the first engine starting system to alternately accelerate and decelerate the first engine during motoring with respect to the second engine based on one or more adjustments to a baseline dry motoring profile to shift timing between the first engine and the second engine.
 2. The system as in claim 1, wherein control of the first engine starting system is based on one or more of: an ambient air temperature, an ambient pressure, an oil temperature, engine drag, and parasitic factors.
 3. The system as in claim 1, wherein a power source of the motoring includes a pneumatic or electric power source from at least one of: an auxiliary power unit of the aircraft, a ground cart, a cross engine supply, or a stored energy system.
 4. The system as in claim 1, wherein the first engine starting system is controlled based on the first controller commanding one or more adjustments to the baseline dry motoring profile to shift timing of use of compressed air to drive a first air turbine starter.
 5. The system as in claim 1, wherein the first controller is configured to coordinate with the second controller to control the first engine starting system and the second engine starting system of the second engine.
 6. The system as in claim 5, wherein the first controller is configured to alternate supplying power from a power source between the first engine starting system and the second engine starting system.
 7. A system of an aircraft, the system comprising: a compressed air source operable to supply compressed air; a first engine system comprising a first gas turbine engine, a first air turbine starter, and a first controller configured to control delivery of the compressed air to the first air turbine starter during motoring of the first gas turbine engine; a second engine system comprising a second gas turbine engine, a second air turbine starter, and a second controller configured to control delivery of the compressed air to the second air turbine starter during motoring of the second gas turbine engine; and at least one engine control interface configured to cause supply of the compressed air to alternate between the first air turbine starter and the second air turbine starter to intermittently accelerate and decelerate the first gas turbine engine with respect to the second gas turbine engine, wherein the at least one engine control interface is configured to coordinate control, via the first controller and the second controller, alternating supply of the compressed air between the first air turbine starter and the second air turbine starter by commanding one or more adjustments to a baseline dry motoring profile to shift timing of use of the compressed air between the first engine system and the second engine system.
 8. The system of claim 7, wherein the system is configured to coordinate control, via the first controller and the second controller, the alternating supply of the compressed air between the first air turbine starter and the second air turbine starter based on one or more of: an ambient air temperature, an ambient pressure, an oil temperature, engine drag, and parasitic factors.
 9. The system of claim 7, wherein the system further comprises a first air starter valve and a second air starter valve, and the system is configured to coordinate control, via the first controller and the second controller, the alternating supply of the compressed air between the first air turbine starter and the second air turbine starter based on one or more of: an engine speed, a starter speed, a starter valve position, a starter air pressure, a rate of speed change, a rate of starter valve position change, and a rate of starter air pressure change.
 10. The system of claim 7, further comprising a first starter air valve configured to control the first air turbine starter and a second starter air valve configured to control the second air turbine starter.
 11. The system of claim 10, wherein the system is configured to coordinate control of the first starter air valve through the first controller and the second starter air valve through the second controller.
 12. The system of claim 11, wherein a first engine control interface is configured to buffer communication of the first engine system from an aircraft level communication, a second engine control interface is configured to buffer communication of the second engine system from the aircraft level communication, and the first engine control interface is configured to communicate with the second engine control interface. 